Carbon Capture Goes Electric

A quiet shift is happening in the carbon capture sector. For years, most machines that pull carbon dioxide out of ambient air, known as direct air capture (DAC) systems, have relied on steam for heat. Heating is a critical step for freeing and collecting the CO2 after it’s captured. But using steam for this is inefficient, and lately several companies have been experimenting with novel electrical approaches.

One company, AirCapture in Berkeley, Calif., uses microwaves to generate heat and in June won a Tencent CarbonX 2.0 award for it. Another, the startup Sustaera, in Durham, N.C., uses resistive heating in its lab-scale machine and in March announced it had reached an efficiency that’s more than three times higher than steam. And London-based Mission Zero Technologies built a membrane electrodialysis machine that bypasses the need for heat altogether.

The efforts follow a tumultuous batch of years for commercial DAC technologies. Companies in this sector aim to remove gobs of CO2 from the atmosphere to slow global warming and sell the credits on carbon markets. But they’re plagued by high costs, inefficiencies, commercial setbacks, struggling carbon markets, and political headwinds in the United States. So far, they haven’t made a dent in removing the greenhouse gas.

DAC “is something we all need to have, but no one wants to pay for, so we need to make it as low-cost as possible,” says Cory Sanderson, CEO and co-founder at Sustaera. “People are pivoting into [electrical approaches] because steam is not going to work, and I think the industry is finally realizing that.”

Steam vs. Electric DAC Efficiency

In a typical steam-based DAC system, big fans draw in ambient air and run it over a bed of sorbent material housed in collector units. The sorbent soaks up CO2, and when it’s saturated the collector units close. Then, steam fills the chambers, heating the sorbent and breaking its chemical bonds with CO2, freeing it for collection—a step called desorption. Zurich-based Climeworks—one of the biggest names in atmospheric carbon capture—does it this way.

“People are using steam because it’s a very mature industrial practice,” says Zeyan Liu, an electrochemist at Northwestern University in Evanston, Illinois. But “we see there’s a trend moving from steam to electricity,” he adds.

That’s because there’s a limit on how efficient steam-based desorption can be. For one, steam fills its containment space, heating up not just the sorbent but everything it touches. Steam also must undergo phase changes—from vapor to liquid and back—and the efficiency of that is limited by thermodynamics. Plus, steam struggles in cold environments.

Electric heating approaches can be more targeted. They heat only the sorbent—or even just the chemical bonds within the sorbent—without heating the rest of the collector unit. That’s why Sustaera chose resistive heating, also known as Joule heating.

In this approach, an electric current flows across a sorbent that is made of a conductive material. As the moving electrons encounter resistance in the material, they cause vibrations. This heats up the sorbent without warming the entire chamber. In Sustaera’s case, the company uses a solid sorbent made from an amino acid salt. “The amount of stuff that has to get heated is significantly less, and the efficiency is much higher, so you can heat it faster and you can put in way less energy,” Sanderson says.

It’s a neat idea with a long way to go. Sustaera’s machine is set up in a cramped cinder block room in a business incubator building in the Research Triangle Park area of Durham. It pulls in ambient outdoor air through a hole in the wall. About half of the room is taken up by a rack of tubes used for testing sorbents, and the other half by the DAC machine, nicknamed Phoenix. The machine is capable of capturing about 1 tonne of CO2 from the air per year. (For a frame of reference, the pine forest that surrounds the building will take up far more CO2 than the machine.)

One of the challenges of the technique is making a sorbent that won’t degrade from the heat and that conducts electricity uniformly. Poor design can result in hot spots, degrading the sorbent with each use. When IEEE Spectrum visited Sustaera’s lab in July, Sanderson was testing four sorbent samples, each the size of a wine cork. “We screen the sorbents and any new structured materials here first. And we can run it for thousands of cycles [to get] data that shows our stability,” Sanderson says.

Resistive Heating for Carbon Capture

Resistive heating strategies for DAC, like Sustaera’s, are promising enough that the aerospace giant Airbus filed a patent for it in Europe. An Airbus spokesperson confirmed that the company “has researched the electrical desorption technology described in the patent” as part of its carbon capture R&D, but did not elaborate.

Other groups are focused on an even more targeted heating approach: microwaves. In this method, microwaves selectively break the bonds between CO2 and the sorbent, freeing them for collection.

AirCapture is developing this method. It works like this: Air is drawn in with fans, and the carbon dioxide from that air is absorbed by the sorbent. To heat up the bonded CO2, microwaves are beamed at it. CO2 is transparent to microwaves, but when it’s bonded to a sorbent, the bonds look like water. The microwaves energize those bonds, generating heat and freeing the CO2 molecules so that they can be collected. AirCapture built its DAC machine on a pilot scale at its facility in Berkeley and has been running it for about a year.

“[DAC] is something we all need to have, but no one wants to pay for, so we need to make it as low-cost as possible.” —Cory Sanderson, Sustaera

But the company is also continuing to pursue its more established steam-based approach. Matt Atwood, founder and CEO at AirCapture, anticipates applications for both microwaves and steam. Using steam will cost less in settings where it’s readily available, such as from industrial waste heat, he says. In other environments, microwaves will make more sense.

“For example, if we were developing something in a place where geothermal steam was basically free, then steam is going to be better than microwaves,” Atwood says. “But in most cases, if we’re paying [for power] to generate the microwaves or paying to generate the steam, microwaves will generally be lower cost because it’s lower capital, less equipment, and more energy efficient.”

Electrochemical Direct Air Capture

But a lot of testing will need to be done before anyone can give a real cost comparison between steam and microwaves, says Liu at Northwestern. “There’s not much research on [electrical heating] in DAC,” while electricity consumption in making steam “is a widely validated number,” he says.

At Northwestern, Liu is focused on an electrochemical route, which unlike other options, doesn’t rely on heat. Instead, an electric current drives a chemical reaction that directly couples with the CO2, breaking the chemical bonds that hold it. Companies such as Mission Zero Technologies and Israeli startup RepAir are taking this approach.

No matter which method a DAC company takes, they’re facing an uphill battle against funding, reputation, and gatekeeper pressure. CarbonCapture in 2024 scrapped its plans to build a facility in Wyoming after it couldn’t secure enough carbon-free energy for the project. Project Cypress, a planned DAC hub in Louisiana that involved both Climeworks and Heirloom Carbon, was delayed for months by political attempts to cancel its federal award money. Plans resumed in April this year.

And Microsoft, which almost single-handedly established the carbon market in the U.S., reportedly told companies in April it was pausing its purchasing of carbon credits, but then a month later bought some more.

The mixed signals put pressure on companies to perform. “It’s definitely tough,” Sanderson says. The work is still being done but there’s “a culling of the herd…like, okay, let’s actually see who’s the best.”